In the field of electronics for vehicle applications an improvement is needed to maintain the operation of various electronics during the transition event of engine starting. Typical prior art power systems would ensure electronic sub-systems are maintained in an off or standby condition during engine starting. This was done to ensure that voltage transients from the engine starter do not adversely affect sensitive electronic components. Additionally, during engine starting, via the engine electric starter, the available battery voltage would fall significantly. This reduction in battery voltage is likely to cause connected electronics to go through a reset or start-up condition if powered during engine starting. Additionally, when the engine starter is de-energized at the end of the cranking cycle it is likely that stored energy in the magnetic field of the starter motor will release voltage transients on the associated bus voltage supply. The voltage transients from the electric starter motor may damage electronics that are connected and powered by the associated bus voltage supply.
The system described in this disclosure resolves the issues resulting from bus voltage supply reduction as well as voltage transients during engine cranking. The system of this invention allows electronic equipment to be safely utilized and operational during engine cranking. Additionally, the system described eliminates the need for a back-up or standby battery that typically would be used to separately power typical electronic equipment during engine starting.
Traditional vehicle power supply systems utilized on general aviation aircraft, automobiles and boats often consists of a twelve volt lead-acid storage battery and an engine driven alternator configured to produce a dc voltage through a series of rectifiers. Electrical power is provided to the vehicle system electronics and other electrical loads through a bus system of circuit breakers or fuses, wires, switches and relays. During normal vehicle operation the bus supply system provides power at a nominal bus voltage to the connected electrical loads. When the vehicle engine is not running, no power is produced by the vehicle alternator; therefore, all electrical loads are powered by the storage battery. Typically, this battery will have a nominal voltage of twelve volts dc and will have a reduced voltage as the battery is discharged by the connected loads. The typical lead-acid battery will deliver energy to the connected loads until it is exhausted or until the electrical loads stop requiring energy. Some electronic loads such as radios or GPS navigation systems require ten volts or more to remain operational. If the supplied battery voltage falls below this level the electronics will automatically shut off.
The storage battery used in this type of system is recharged by the engine driven alternator. When the engine is operational the attached alternator produces electrical power, typically at a voltage of between thirteen to fourteen volts dc. This voltage is sufficiently high to recharge the associated vehicle storage battery as well as operate the electrical loads connected to the bus supply system.
One of the associated electrical loads on a typical vehicle electrical system is an electric starter used to start the associated engine. The electric starter is typically a heavy duty DC motor which is intermittently connected by a gearing mechanism to the engine flywheel. During engine starting, the starter motor is operated and the gearing mechanism engages the motor to the engine causing the engine crankshaft to spin and the engine to start. This process is often referred to as engine cranking. During engine cranking the DC motor based electric starter may draw hundreds of amps of current to cause the engine crankshaft to spin at a sufficiently high speed to enable engine starting. The high current required during this starting phase of operation comes from the lead-acid storage battery in the vehicle. This high starting current causes the battery voltage to drop considerably. The resistance of various elements in the bus supply system are subject to the high starter current and thereby have an associated voltage drop as dictated by ohms law, V=I*R. The voltage drop in the system is equal to the product of the current and the associated resistance. Typically, the battery internal resistance, the supply cable resistance and the main starter contactor resistance all add to a total resistance through which the starter current must flow. In a general aviation aircraft there often is a master contactor as well that the starter current must flow through. As an example, if the total system resistance had a value of twenty milliohms and the starter current where two hundred amps then a system voltage drop of four volts could be expected. This voltage drop of four volts would be subtracted from the system bus supply voltage, thereby resulting in an effective supply voltage of about eight volts. During the engine cranking period the reduced supply voltage, in this example eight volts, would be provided to all connected and powered electrical loads.
At the end of the engine cranking period another electrical phenomena occurs on the power bus supply. The large current flowing through the system may generate a high voltage transient as the current through the starter is discontinued. The collapsing magnetic field in the starter motor will generate voltage transients. During the time period when the starter contactor is becoming non-conductive, those transients will be coupled to the power bus supply. Additionally, the power bus supply wiring will have an associated wiring inductance. The rapid change in starter current during the cranking period will also generate a voltage transient due to the changing current in the power bus supply wiring due to the wire inductance. These voltage transients will be applied to all connected electrical loads in the system.
During the engine cranking period the reduced supply voltage has various effects on the connected electrical loads. Electrical loads such as incandescent light bulbs will operate at a reduced wattage and appear dimmer than normal. Electrical loads such as DC motors will tend to operate at a slower speed. Electrical loads with electronic components, however, may have a different reaction to the reduced bus supply voltage. Often electronic loads will have a drop-out voltage, below which they will stop operating. Typically, electronic equipment designed for operation on a nominal twelve volt bus system will operate normally down to ten volts, below which they turn off. The turn off below ten volts may be associated with a long restart time after the bus voltage is restored to a value above the drop-out voltage. In general aviation applications this time period may be on the order of minutes and may be highly undesirable.
In general aviation aircraft there are a number of electrical loads which are preferably operational before, during, and after engine cranking. For example an electronic engine monitoring system that displays engine rpm and engine oil pressure would preferably be fully operational through these stages of engine operation. Unfortunately, many engine monitor systems have microprocessors and software based operating systems that can take minutes to boot-up upon the application of power above the drop-out voltage level. Starting an aircraft engine without immediate knowledge of oil pressure is highly undesirable.
Another electronic system that is adversely affected by low bus supply voltage is general aviation GPS systems. These systems typically require a few minutes to boot-up and acquire adequate satellite coverage to allow for proper navigation. Additionally, many GPS systems include user programming memory for active flight plan storage. The active flight plan is stored in volatile memory and subsequently erased during power down conditions. It is highly desirable to have the GPS system up and running prior to engine starting, particularly to provide the user an opportunity to make flight plan changes in the GPS memory without wasting fuel running the engine. However, unless an alternate source of power is available, the GPS will go off during engine cranking, thereby loosing active flight plan information.
The system of the invention allows electronic equipment to be operated from the vehicle bus supply through all phases of engine cranking by boosting the equipment supply voltage when necessary (ie. during engine cranking). The system may be used to raise equipment supply voltage back to a nominal voltage of approximately twelve volts even when the battery voltage falls to approximately four volts. The system of the invention may additionally include transient voltage suppression means such as MOVs or transorbs to absorb potentially harmful voltage transients.